Background information on technology and terms can be found in the IEEE 802.3 specification (particularly clauses 64 and 65 for 1 gigabit [1 G] and the to-be-released clauses 76 and 77 for 10 gigabit [10 G]). The need for redundancy in networks, and in particular in Ethernet passive optical networks (EPONs) is known in the art. Referring to FIG. 1, a diagram of an EPON network, an OLT (optical line transmission equipment of the network provider) (100, 101) communicates over a fiber optic network (104A, 104B, and 107A, 107B, 107N) with ONUs (optical networking units associated with a user) 108A, 108B, 108N. A passive splitter 106 facilitates either OLT-0 100 or OLT-1 101 communicating with the ONUs 108A, 108B, 108N. A host 124 provides configuration information, command, control, and monitoring of the OLTs (100, 101). A variety of architectures can be used to provide redundancy of the fiber optic network and the EPON communications standard includes features to facilitate recovery from a network failure. Redundancy in an EPON system facilitates continuity of service including switchover from a first OLT (sometimes called the “main”) to a second OLT (sometimes called the “standby”) and/or from a first network to a second network. This switchover can include alternative OLTs and fiber optic cables.
US publication 2008/0037981 A1 to Toshiaki Mukojima for Optical communication system with N+1 redundancy teaches a system for two or more active interfaces, each controlling the transmission and reception of optical signals between a communication network and one or more subscriber terminals. The control information used by all the active interfaces is stored in a common memory accessible to all of the interfaces. If a fault is detected in an active interface, a standby interface extracts the control information of the faulty interface from the memory and sends the control information to the standby interface. The interfaces are necessarily co-located because the control information includes the RTT (round trip time) data. Also required is an optical switching apparatus that switches data transmission paths among the network, the active and standby interfaces, and the subscriber terminals.
US publication 2008/0025724 A1 to Hirokazu Ozaki for PON system, station side device and redundant method used for the same teaches a system including a plurality of PONs, a plurality of ONUs, and a station side device including a plurality of PON interface sections using one or more optical switch devices to provide 1:N redundancy of the PONs by cascading the N optical switch modules. The PON interface sections are co-located in the station side device and it is assumed that a value of an adjusted optical channel length inside the station side device is set.
US publication 2007/0268818 A1 to Youichi Sugihara for Station side transmission unit, operation control method for station side transmission unit, and optical networking using station side transmission unit teaches a method for redundant units closing and releasing closure of their interfaces to switch between active units for a PON network. While this method provides physical layer switching, this method does not address the service or operational aspects of network failure and recovery.
In the context of this document a failure includes a problem with the OLT (100, 101), such as an internal logic failure or general power failure, or a problem with the fiber optic network, such as a break in the fiber optic cable 104A, 104B between an OLT and a first node 106 on the fiber optic network. Many conventional redundancy techniques provide switchover from a main OLT to a standby OLT, and after a delay, service is restored to the ONU in accordance with the established specifications. OLT redundancy in EPON systems is relatively simple when the switchover time can be relatively long, approximately 1 second. In this case, the ONUs deregister, the standby OLT comes online, all the ONUs re-register, and service is eventually restored to all of the ONUs.
An ONU will deregister under certain conditions, including:
1. A long time, normally greater than 300 msec, has passed without the ONU receiving any downstream GATE packets.
2. There is a timestamp drift of greater than 16 TQ (256 nsec) in the timestamp received from the OLT by the ONU (1 TQ, or Time Quanta is 16 nsec). A timestamp drift can occur when switching from a first OLT using a first clock to a second OLT using a second clock, where the clocks are not synchronized. Even with synchronized clocks, a difference in the lengths of the fiber optic cable from each of the OLTs to the first node on the fiber optic network is a source of timestamp drift.
3. A new OLT not having the current security key for an ONU. When an OLT sends data using a security key that an ONU is no longer using, the ONU will not recognize the data. Because the ONU does not recognize the data, the ONU will be in a state of not receiving data, and after a given time has elapsed without the ONU receiving data, the ONU will deregister. In a case where the control packets are not encrypted, the ONU will still not be able to recognize the data, and will be in a state of not receiving data. In the context of this document, data is used as a general reference to all information communicated, unless otherwise specified. In a case where the data that the system secures is only the data packets, but the control packets are sent unencrypted, the ONU will similarly be in a state of not receiving data.
When an ONU deregisters, the conventional time to recover is approximately 1 second and typically between 300 msec and 1500 msec. This conventional time to recover is no longer sufficient for new applications being deployed and applications planned for future deployment. Application providers are now demanding a time to recover of less than 50 msec in order to provide new applications and a higher level of service to users. To achieve recovery within this timeframe, the ONUs cannot be allowed to deregister when switching to a redundant OLT.
Conventional redundancy implementations typically co-locate the redundant OLTs in order to share common resources, synchronize OLTs, or minimize differences in the length of fiber optic cables. A popular implementation of OLTs is as a printed circuit board, with multiple OLTs deployed in a single chassis at a network provider's facility and a single host controlling the OLTs. However, in a case where each OLT has a clock generator for the OLT's timestamp counter, the timestamp counters are not synchronized. The timestamp counters advance every 16 nsec (1 TQ), and synchronization requires at least 2 wires (1 wire for clock and 1 wire for reset) connected between the OLTs. This case is a difficult implementation with strict physical limitations. There are variances in the round trip delay (RTT) due to the different lengths of the fiber optic cable from each of the OLTs to the first node on the fiber optic network. In this case, synchronizing the RTT is a manual process that again has strict physical limitations. In addition, co-locating OLTs and hosts exposes the system to failure due to lack of location redundancy. In the context of this description, location redundancy refers to deploying portions of the system at physically diverse locations. In one non-limiting example of location non-redundancy, a main and a standby OLT are both located in the same room. A natural disaster, such as flooding, an accident, such as a fire, or power shutdown for routine maintenance affect both the main and standby OLTs resulting in a failure of the EPON.
Conventional techniques for synchronizing security keys include maintaining a real-time database of the current security key for each ONU. As stated earlier, if a new OLT does not have the current security key for an ONU then the ONU will not recognize the data being sent from the new OLT, potentially causing the ONU to deregister and the data to be discarded. Synchronizing security keys at real time between two OLTs is a complex technique, possibly requiring extensive hardware wiring between OLTs.
It is thus desirable to have a system for EPON redundancy that provides recovery in less than 50 msec. It is further desirable to have redundancy of multiple layers of the EPON, including network provider OLT (equipment redundancy), the fiber optic network (path redundancy), and physical location (location redundancy).